On the spectrum is technically correct as long as it can be implied from an earlier specification that it is about the meant spectrum. And even if it is technically correct, having a good reason for continuing with simply the spectrum would… make sense. If people would realize that they try to leave out the terms autism and autistic for a wrong reason (and maybe they don’t) that would be a success. It’s not about choosing whether something is offensive to me or not, but whether it is, be it intended or not. I am aware that people don’t usually use it to purposefully be offensive, and in that sense I can understand it - but that doesn’t change that (depending on the unconscious reason) it is offensive anyway.
I am sorry if I gave the impression that I thought he definitely wasn’t one, I personally just think that this is one of those things that can’t be 100% certain anymore, even if it may be 99%.